For an electric kart, what is the relationship of torque and power?

Electric cars, trucks, ATVs, NEVs - things bigger than a motorcycle.
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Bmr4life   1 mW

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Joined: Apr 02 2016 10:00am

For an electric kart, what is the relationship of torque and power?

Post by Bmr4life » Sep 10 2019 6:23am

So I've have some good luck with my Razor Dune Buggy adult conversion. The Chinese 48v brushless setup has been tested at up to ~68v and the controller was modified to push ~50 amps. It currently propels the 300lbs of kart and rider to 30mph while still having good torque for getting up hills.

I'd like to build another kart that has two of these motors for dual single wheel drive partly because I drive on pavement a lot and want to turn better and because I want a higher top speed. My batteries can handle well over 100 amps so the capacity is there. This would take me from ~3400w to ~6800w. I need help with determining what the new gear ratio should be so that acceleration remains the same. With that info, I can then figure out the the new potential top speed.

I've seen various formulas online, but all of them make what I'm doing seem impossible with the power I have.
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major   10 kW

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Location: NW Ohio, USA

Re: For an electric kart, what is the relationship of torque and power?

Post by major » Sep 10 2019 7:45am

Power = speed times torque.

Watts = Radains per second * Newton meters
(w = rad/s * Nm)

or

Horsepower = Revolutions per minute * pound feet / 5252
(hp = RPM * lb.ft./5252)

Relationship is true for all rotational power including karts.

Regards,

major

Punx0r   100 GW

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Re: For an electric kart, what is the relationship of torque and power?

Post by Punx0r » Sep 10 2019 7:50am

If you're running 100A into two motors instead of 50A into one you will have exactly double the torque as before, so you can halve your gear ratio to achieve the same acceleration up to 30mph. Your top speed won't double because air resistance goes up a lot as speed increases.

If the calculators you've used are saying don't have enough power to do what you're doing, they might be assuming a more car-like vehicle with a lot more rolling resistance and drag area. Your little cart probably only has similar drag to an upright cyclist.

Bmr4life   1 mW

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Re: For an electric kart, what is the relationship of torque and power?

Post by Bmr4life » Sep 10 2019 10:40am

Thank you both. I've ordered the 60v versions of these motors and controllers so I've not over-volting as much. Plan is 18s, either lipos or hybrid batts.

Quinc   100 W

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Re: For an electric kart, what is the relationship of torque and power?

Post by Quinc » Sep 10 2019 11:32am

This calculator has been spot on for me: https://www.bmikarts.com/Go-Kart-Gear-R ... Calculator

With two of those boma 60v motors you will have a ton of torque so should be able to gear it pretty light. For a speed controller you should look into something like the Kelly 300-400amp brushless that you can configure with your laptop.

Bmr4life   1 mW

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Joined: Apr 02 2016 10:00am

Re: For an electric kart, what is the relationship of torque and power?

Post by Bmr4life » Sep 10 2019 2:42pm

I also use the BMI speed calculator, it has been spot on for me as well. Once I know the gear ratio I'm going to use then I'll use it to get theoretical top speed.

As for the controller, I'd have to use two since these are brushless motors. Also, I already have a large 400amp Kelly controller and a 16kw Motenergy motor. I was going to put them on a different cart chassis but I now realize that setup is overkill in many ways (big, heavy, overly powerful).

My goals are very unique
  • 45mph top speed
    Can to run on pavement or dirt (rules out race kart chassis)
    Can easily make tight turns on pavement at low speeds
    Can fit through my 36" basement door (either drive in or carry)
    Doesn't weigh a ton (Razor Kart is 110lbs ready to run)
The Razor Kart was a good first stab and I've learned a lot from using it. Lots of people doubted the 48v motor setup, had me thinking I would need two just to hit 25mph.

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